The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art to the present technology.
New and improved dyes are being used for a wide variety of applications in dyeing and printing industries, textile industries, paper and ink manufacturing industries, cosmetics, and food and pharmaceutical industries. Textile dyes or fabric dyes consume nearly two thirds of the total dyes manufactured. Lately, the demand for reactive dyes has increased owing to their bright colors and improved stability. Reactive dyes bind to the substrate by addition or substitution mechanisms under alkaline conditions and high temperature, usually conducted as a batch process. During the batch dying process, a significant fraction of the dye is hydrolyzed and released. Moreover, at the end of the dying process, excess dye is rinsed off using large quantities of water, usually in a multistep process, at boiling temperatures. Consequently, both dye manufacturing and dye consuming industries produce wastewater which is highly colored, containing excess dye, high levels of electrolytes, toxic substances (e.g., metals and unreacted raw materials) and auxiliaries. This wastewater has high pH and poses unacceptable environmental risks and severe effluent treatment problems.
Ideally, fixation of the dye to the substrate is 100% efficient, that is every molecule of dye supplied to the substrate is fixed so that none appears in a waste stream. It is desirable: 1) to achieve a fixation rate as high as possible, using a minimum of associated chemicals and water; and 2) to efficiently remove excess dye, using a minimum of associated chemicals and water.
One of the most popular approaches for dyeing is to use a reactive dye, where a covalent bond between a substrate and a dye is created when the system pH is raised. For perfect fixation at this high pH, reactive groups on the substrate surface must outcompete water for these reactive sites on the dye. Dye molecules that react with water are inactivated and are no longer able to bond covalently with the substrate and must be rinsed away. As a result, dyes are commonly designed to be hydrophobic to increase their tendency to associate with the substrate (compared with water) and to slow the overall kinetics of reaction with water. However, perfection is not attainable and a fraction of the dye will usually hydrolyze rather than react with the substrate surface and must be rinsed away. Under these conditions, the dye's hydrophobicity turns from an asset to a liability because hydrophobic dyes require a large amount of water to be rinsed off completely.
The optimization of a typical dye application process takes into account several opposing parameters, especially related to waste disposal, which can significantly impact the total process cost. For example, during the dyeing process, a more hydrophobic molecule is preferred, as this will cause it to associate with the substrate rather than with water and, therefore, improve fixation efficiency and reduce overall wastes. However, an increase in the dye fixation as a result of increased hydrophobicity is often accompanied by the drawback of requiring much larger water volumes to remove unfixed dye after treatment. Current solutions must find a compromise between dye performance and its ease of removal and, therefore, fail to optimize either attribute.